Oral cancer has been associated with the oral strain of herpes simplex virus, HSV-1, by small-scale studies of cell-mediated immunity. Cancer of the uterine cervix has been closely associated with the genital strain of herpes simplex, HSV-2, by studies of antibodies of the virus. Serum will be collected from patients with oral cancer or leukoplakia attending an Oral Medicine clinic, and will be examined for HSV-1 antibody by measurement of the neutralization constant. An increased activity of HSV-1 antibody will be sought in patients of each group, as compared with controls matched for age, sex and smoking habits. The immunoglobulin class of antibodies to herpes simplex virus will be examined in patients with cancer, leukoplakia, primary herpetic stomatitis or recurrent herpes labialis and in control subjects. The hypothesis that after an acute herpetic infection, only antibody of the IgG class persists, and that appearance of antibody in other immunoglobulin classes is evidence of a recurrent infection, will be tested. If sera from cancer patients contains herpes simplex antibody of non-Ig class, this would be evidence for a continuing stimulus by herpes simplex antigens, from the tumor. Sequential studies will determine whether an increased antibody neutralization constant in patients with leukoplakia indicates an increased risk of transformation into carcinoma. Patients with oral cancer will be observed repeatedly to find if a high or rising neutralization constant indicates a good prognosis, as has been reported for cervical cancer.